The present invention relates to a printing plate and method of making it, and more particularly to such a printing plate having a relief image formed on its surface by digital imaging photopolymerization.
Flexography or flexographic printing are terms which have been applied broadly to a specialized form of relief printing which utilizes rubber or other elastomeric relief plates. Because the inks used for flexography dry very quickly, flexographic printing has been widely used to print a variety of nonabsorbent surfaces including plastics and foils as well as thick substrates such as paperboard. More recent improvements in the quality of the rubbery relief plates used in flexographic printing has broadened the types of jobs which are printed using this technique.
The rubbery relief plates used in flexography have been made in the past by casting or pressing a rubber sheet against a metal negative relief surface and then hardening the rubber. More recently, the plates have been manufactured using photohardenable polymeric compositions. Typically, a sheet of the photohardenable composition is covered with a flexible polymeric film and imagewise exposing the composition through the film using actinic radiation. The polymeric film and unhardened portions of the composition are then stripped away and removed, leaving the imaged relief surface.
However, problems have remained with flexographic printing plates including loss of sharpness in the relief surface caused by the diffraction of the actinic radiation passing through the film layer or mask to the surface of the photohardenable composition and distortion of the rubbery relief surfaces when the plate is mounted onto a plate cylinder for printing. Additionally, as the plates are customized for individual repetitive printing jobs, the printer must maintain a large inventory of the plates typically mounted on a replaceable sleeve, which takes up valuable plant space and increases parts costs to the printer.
Accordingly, needs remain in this art for a printing plate which can be accurately imaged and readily replaced to provide improved print quality while lowering costs to the printer.
The present invention addresses these needs by providing a printing plate having a relief imaged surface which is reimagable, i.e., the relief image(s) can be removed and new images replaced on the surface. The plate may be utilized in flexographic printing processes. Further, with suitable modifications, the plate may also be useful in other direct and indirect relief printing processes including offset lithography as well as in intaglio processes such as direct and indirect gravure printing processes. The printing plate of the present invention may be provided as either a cylindrical or flat printing plate.
In accordance with one aspect of the invention, a printing plate is provided which comprises a support assembly and a relief-imaged surface on the support assembly. The support assembly may be a composite of one or more layers of metal, fabric, and/or polymer to provide requisite properties of compressibility and conformability as well as a stable support surface for the raised image. The support assembly may take a variety of forms. In one embodiment, the support assembly comprises an expandable, cylindrical sleeve including a base layer, at least one compressible layer, and a surface layer which includes a relief image. The sleeve is replaceable and can be mounted and demounted onto a metal cylinder or roller using pressurized air. The support assembly is constructed to provide an inner diameter and shape which is mountable onto a cylinder or roller without slippage during the printing process. Alternatively, the support assembly may comprise a flat polymeric base layer including the relief imaged surface, which base layer can be used in either a flat configuration, or mounted onto a cylinder for printing.
The relief imaged surface is formed using digital imaging photopolymerization techniques in which a liquid photopolymer layer is provided on the support assembly and is exposed to a light source which emits radiation for a time sufficient to cure and harden the photopolymer in areas exposed to the radiation. The term xe2x80x9cphotopolymerizationxe2x80x9d is meant to include both polymer formation and polymer crosslinking and also includes photohardenable compositions. Preferably, a raised relief image of approximately 0.40 to about 1.0 mm is provided. The liquid photopolymer may be any light-sensitive polymer which will cure and harden after exposure to actinic radiation. The photopolymer is preferably selected from the group consisting of acrylates, epoxies, urethanes, and unsaturated polyesters.
In one embodiment of the invention, the relief imaged surface is formed at a digital imaging station using stereolithography techniques. At the imaging station, the support assembly, in the form of a cylindrical sleeve, is immersed into a bath of a liquid photopolymer or meterly coated with liquid polymer in a non-immersive manner. The support assembly is rotated so that successive portions of the support assembly which are covered by the photopolymer are exposed to a source of actinic radiation such as a laser which is translated along or across the surface of the support assembly. Alternatively, the laser may be fixed, and the support assembly translated and rotated appropriately. Those portions of the photopolymer which are exposed to the laser cure and form raised image areas. Laser scanning is repeated until the entire surface is imaged.
The resulting printing plate may then be mounted onto a press and used to print in a conventional manner. Once the particular printing job for which the image was produced has been completed, the imaged sleeve may be demounted and stored for later reuse. Alternatively, the relief image surface may be removed at a reprocessing station, and the surface of the plate prepared for receiving a new image at the digital imaging station. The relief image is preferably removed by an abrading mechanism which mechanically grinds, scrapes, or otherwise cuts away the imaged areas until the surface of the support layer is exposed. Alternatively, the image may be removed by chemically dissolving and washing away the raised image.
Because in a preferred embodiment of the invention, the plate is in the form of a replaceable sleeve, the printer need not tie up a printing cylinder for each plate; the sleeves may be readily demounted and stored. Further, as the image on the printing plate is replaceable, the printer need not maintain a large inventory of plates. This reduces costs. Lastly, as the image is formed digitally from a computer-stored file, there is no degradation in quality of the image as with masks or film layers so that the image which is printed is sharp and well defined.